This invention relates to papermachine clothing, particularly, but not exclusively to clothing for use in the press section of a papermaking machine.
WO 92/17643 discloses papermachine clothing of the kind for use in the press sections of a papermaking machine which includes a base fabric composed of superimposed layers of synthetic thermoplastics material in mesh form which are secured together. The superimposed layers have apertures of different respective sizes and the base fabric formed by the superimposed layers provides support and reinforcement to a fibrous batt. The lower (machine contacting) layer may have a coarser mesh than the upper (batt contacting) layer.
It has been found that when resilient membranes are used, as at present, in such structures, the flexibility of the material under the pressures exerted in press nip causes the membrane material to deform and thus reduce or even close the mesh apertures. This in turn creates problems with re-wetting of the paper web with water which has entered the mesh being squeezed back through the batt into paper from which it has previously been extracted.
An object of the invention is to provide papermachine clothing with improved dimensional stability and strength, whereby the tendency towards closure of mesh apertures under nip pressure is much reduced and rewetting, and marking, of the paper web alleviated.
In accordance with the invention, papermachine clothing includes a base fabric comprised of at least two superposed membrane layers of synthetic thermoplastics material in mesh form, characterised in that the lower (machine side) membrane layer consists of a harder material than the upper (paper side) membrane layer which has a lower maximum creep modulus than the lower membrane layer.
Preferably, said upper membrane layer is of a thermoplastic polyurethane matrix material, with a Shore A hardness of, for example, between 65 and 85. The lower membrane preferably comprises a polyamide matrix material with a Shore D hardness of, for example, between 40 and 45.
A sandwich layer of batt staple fibre may be provided between the membrane layers.
The base fabric will normally in use support a fibrous batt of known form and materials, for example as described in WO 92/17643.
The effect of using a very hard lower membrane as envisaged is to create a large number of pockets into which water expelled from the paper sheet can be accepted. This can significantly increase the dewatering capacity of the felt.
The base fabric may of course be composed of three or more superimposed layers of perforated membrane and these may increase in hardness from the uppermost to the lowermost layer and similarly the sizes of the apertures may increase from the uppermost to the lowermost layer. Preferably at least one membrane layer contains parallel reinforcing yarns extending in at least one direction, and the fabric may comprise two such layers. These membranes may be made in accordance with GB-A-2254288. The base fabric may contain one or more woven base cloth layers either between or on an outer face of the membrane layers.